In commercial container filling or packaging operations, the containers typically are moved by a conveying system at very high rates of speed. Dilute aqueous lubricant compositions are typically applied to the conveyor or containers using spray or pumping equipment. These lubricant compositions permit high-speed operation of the conveyor and limit marring of the containers or labels. One problem that can occur with thermoplastic beverage containers made from polyethylene terephthalate (PET) is environmental stress cracking. Stress cracking in polymers is the development of cracks normal to an applied stress as a result of stress promoted chemical degradation. Typically amorphous polymers are more susceptible to stress cracking. In the case of PET, it is the amorphous regions of a beverage container such as the center of the base of a PET bottle that are most susceptible to stress cracking. When stress cracks penetrate through the wall of a PET bottle, the bottle fails either by leaking or bursting. Because of environmental stress cracking, bottles filled with carbonated drinks are at risk for failure, especially at elevated temperatures (e.g., warmer weather, elevated storage temperatures, etc.). The risk of environmental stress cracking is exacerbated by the presence of materials which are incompatible with PET. Materials that, when in contact with PET increase the rate of occurrence of environmental stress cracking are considered incompatible with PET while materials that result in no increase in environmental stress cracking are considered compatible with PET. The failure rate of PET bottles is greater for bottles that have been contacted with alkaline water than for bottles that have been contacted with deionized water, thus it can be stated that the presence of alkalinity decreases the compatibility of aqueous compositions with PET bottles.
It is often the case that water used in the preparation of conveyor lubricant compositions contains alkalinity. For example, the alkalinity of water used for dilution of conveyor lubricants in bottling plants typically ranges between about 10 ppm and 100 ppm, expressed as ppm of CaCO3 (calcium carbonate), with occasional values above 100 ppm. According to the International Society of Beverage Technologists web site, it is strongly recommended to keep the total alkalinity level (expressed as CaCO3) below 50 mg/L (equivalent to 50 ppm as CaCO3) in the water used to dilute lubricant concentrate compositions (lube make up water) in order to minimize the risk of stress crack failure. It is therefore important for conveyor lubricant compositions to show good compatibility with PET beverage bottles in the case that the dilution water contains alkalinity, particularly in the case that the dilution water exhibits alkalinity levels above 50 ppm and up to and in excess of 100 ppm, measured as CaCO3.
Silicone based lubricants are preferred lubricants for PET bottles because they provide improved lubrication properties and significantly increased conveyor efficiency. Silicone containing lubricant compositions are described, for example in U.S. Pat. No. 6,495,494 (Li et. Al which is incorporated by reference herein in its entirety). However, aqueous silicone based lubricants may be considered to be less compatible with PET than other types of lubricants such as phosphate ester based lubricants. For example, conventional aqueous silicone lubricant compositions generally show a relatively higher incidence of stress cracking under conditions of high alkalinity. There has therefore been an unmet need in the field of conveyor lubrication which is an aqueous silicone conveyor lubricant that exhibits good compatibility with PET, particularly in the case that the lubricant contains alkalinity, for example from the dilution water.
It is against this background that the present invention has been made.